1. Field of the Invention
The present invention relates to a flat panel display device having a digital memory provided in each pixel and a method for driving the same.
2. Description of Related Art
In recent years, a liquid crystal display device has been widely and increasingly adopted in the field of compact information terminals such as a portable telephone and an electronic book, because it advantageously is light, thin and consumes low power. Such compact information terminals are typically driven by a battery and therefore, it strongly requires a configuration that consumes low power.
Particularly, a portable telephone needs to have the ability to perform display functions while consuming lower power. To achieve such a portable telephone, a technique is disclosed, for example, in Japanese Patent Application Laid-open No. 2001-264814. The publication discloses a liquid crystal display device including a digital memory provided in each pixel, in which when a still image is displayed, still image data stored in a digital memory is used to display the image and while operating only an alternate drive circuit for alternately driving a liquid crystal and stopping the operation of other peripheral drive circuits. According to the technique, only an alternate drive circuit operates during still image display period and therefore, the liquid crystal display device is able to operate consuming lower power.
The aforementioned liquid crystal display device is considered to operate such that when the alternate drive circuit is made to operate, the rise time of a memory control signal used to control a switching element for retrieving data representing a still image from a digital memory and the rise time of a common signal supplied to a common electrode are each made long, reducing electrical power consumed by a signal generation circuit for generating a memory control signal and a common signal.
However, the rise time of a memory control signal or a common signal needs to be kept within a vertical blanking period during which a typical halftone display or a typical moving image display (hereinafter, referred to as a typical display) is switched to a still image display. This is because if the rise time of a memory control signal or a common signal becomes longer than a vertical blanking period, an erroneous write operation occurs while still image data is written to a digital memory. As noted above, the rise time of a memory control signal or a common signal is not allowed to become longer than the vertical blanking period, and the problematic nature of present day technique occurs, in particular when a display device further reduces power consumption during still image display operation.
To avoid the above-mentioned problem, a configuration for making a vertical blanking period longer in response to lengthening the rise time of a memory control signal or a common signal may also be conceived. In this case, erroneous writing of still image data can be avoided, but the typical display period becomes short and a clock frequency at which a write operation is performed during the typical display operation becomes high, leading to an increase in power consumption. Accordingly, such a configuration cannot provide an effective solution to the aforementioned current problems.